The present invention relates to a light scanning device.
Known light beam deflecting means include a rotating polygonal mirror and a hologram scanner. While the polygonal mirror or the hologram scanner is making one revolution, the light beam is deflected a plurality of times by a plurality of mirror surfaces or hologram gratings. Since light beam deflection is effected by such mirror surfaces or hologram gratings in the polygonal mirror or the hologram scanner, these known light beam deflectors have suffered the problem of irregular deflecting surfaces which do not lie parallel to each other. To correct such irregular reflecting surfaces, a complex optical system has to be incorporated or the reflecting surfaces of the polygonal mirror have to be fabricated with high precision.
In view of the above drawback, there has been proposed a scanning means comprising a rotatable reflecting medium having a mirror surface or surfaces which are inclined with respect to the axis of rotation of the rotatable reflecting medium. A light beam to be deflected is applied along the axis of rotation of the rotatable reflecting medium to the mirror surface or surfaces, which are rotated to deflect the reflected light beam. The mirror surface or surfaces in the scanning means are disclosed in:
(i) Xerox Disclosure Journal, Volume 8, Number 6, November/December 1983,
(ii) SPIE, Vol. 200, Laser Recording and Information Handling (1979)/119.
In the disclosed arrangements, only one or two mirror surfaces are employed, and such a few mirror surfaces can easily be fabricated. No problem of irregular, out-of-parallel reflecting surfaces is experienced by a single mirror surface.
No difficulty arises if a light beam applied to a mirror surface is of a circular cross-sectional shape. However, if the cross-sectional shape of an applied light beam is not circular, but elliptical, for example, the cross-sectional shape of the reflected light beam necessarily varies in the directions of the major and minor axes of the elliptical shape dependent on the angular position of the mirror surface.
When a mirror surface inclined at 45.degree., for example, to the axis or rotation is rotated, the reflected beam appears as if rotated for each of illuminated positions on the surface being scanned in a main scanning direction. In the position where the mirror surface has rotated 90.degree., the major and minor axes of the elliptical shape are positionally inverted or switched around between the main and auxiliary scanning directions.
Where a light scanning device is composed of such a scanning means having a mirror surface or surfaces and a semiconductor laser as a light source, a light beam of an elliptical cross section is emitted from the semiconductor laser since the light beam diverges at different angles in the direction of a junction surface and a direction normal to that direction. As the elliptical light beam cannot easily be corrected even by a lens, a prism, or the like, the diameter of a final beam spot on a surface being scanned becomes irregular, and hence the quality of images reproduced by a write system such as a printer may be lowered.
A laser printer has a focusing optical system including an f.theta. lens, a toroidal lens, and the like and disposed in the path of an emitted light beam. The diameter of a focused light beam is usually determined by the diameter of a light beam applied to these lenses.
Inasmuch as the beam applied to the lenses has different diameters in the main and auxiliary scanning directions, the final beam spot on a surface being scanned is of different diameters in the main and auxiliary scanning directions.
In a beam scanning device in which an elliptical light beam is deflected by being applied to a mirror surface at a certain finite angle to a direction normal to the mirror surface, especially where incident and reflecting angles are about 45.degree., the elliptical light beam being deflected is scanned while being rotated upon rotation of the mirror surface, and the spot diameter of the converged light beam focused by a focusing lens on a surface being scanned varies in the main and auxiliary scanning directions, thus adversely affecting the image quality.